1. Field of the Invention
The invention relates generally to a method and apparatus for quantifying fluid contamination as an indication of sample cleanup in real time in a wellbore environment. Specifically, the invention is a method and apparatus for measurement of physical properties of fluid being pumped from a formation surrounding a wellbore by a wireline or monitoring while drilling tool to estimate sample cleanup or to predict the time at which a sample having a desired purity can be obtained.
2. Summary of the Related Art
In wellbore exploration, typically drilling muds such as oil-based muds and synthetic-based muds or water-based muds are used. The filtrates from these muds generally invade the formation through the borehole wall to an extent, meaning that this filtrate must be removed from the formation in order to access the formation fluids. Open hole sampling is an effective way to acquire representative reservoir fluids. Sample acquisition allows determination of critical information for assessing the economic value of reserves. In addition, optimal production strategies can be designed to handle these complex fluids. In openhole sampling, initially, the flow from the formation contains considerable filtrate, but as this filtrate is drained from the formation, the flow increasingly becomes richer in formation fluid. That is, the sampled flow from the formation contains a higher percentage of formation fluid as pumping continues.
It is well known that fluid being pumped from a wellbore undergoes a clean-up process in which the purity of the sample increases over time as filtrate is gradually removed from the formation and less filtrate appears in the sample. Here, fp is defined to be the fraction of purity and fc to be the fraction of contamination, where fp+fc=1. As the composition of the sampled formation fluid changes, so do the optical and physical properties of the sampled fluid, such as optical absorption, fluorescence, refractive index, viscosity, density, sound speed, and bulk modulus. A number of different measurements are used to determine various optical and physical properties of a fluid downhole in real time. Measuring these properties of the fluid therefore provides qualitative insight into a fluid sample's purity but does not provide a quantitative value, fp, for the fluid sample's purity. There has been a mistaken notion that, after pumping for a long time, the fraction of fluid contamination necessarily drops to zero. Actually, in many cases where, after a long pumping time, some optical or physical property was not substantially changing yet the fraction of contamination (as subsequently determined in a surface lab) was far from zero and was even as high as 45%. In that case, the terminal purity was only 55%.
At long pumping times, a dynamic equilibrium can be reached in which a fluid sample being withdrawn from a tapped zone cleans up at the same rate that it is being recontaminated from above and below that tapped zone. Thus, even though a downhole measured property (OD, etc.) has substantially stopped changing, the sample is still not at 100% purity. This dynamic equilibrium depends on various factors such as the ratio of the vertical to horizontal permeability. Therefore, we define ftp to be the fraction of the terminal purity, where the terminal purity is the purity achieved at very long pumping times and is usually less than 100%. All that we can estimate by monitoring changes in OD or some other property over time (or over volume pumped) is the fraction of the terminal purity, ftp, but not the fraction of formation-fluid purity, fp.
When extracting fluids from a formation, it is desirable to quantify the cleanup progress, that is, the degree of filtrate contamination in real time. If it is known that there is too much filtrate contamination in the sample (more than about 5% or 10%), then there is no reason to collect the formation fluid sample in a sample tank until the contamination level drops to an acceptable level. On the other hand, if by pumping for a very long time, it is possible to achieve only slightly better contamination level, an operator ends up wasting very expensive rig time and also risks the very costly possibility of allowing a tool to become stuck in the wellbore. Thus, there is a need to determine how long one must pump to obtain a suitable purity sample from the formation.
When pumping first begins, the fluid being pumped contains a large amount of mud filtrate contamination and the fluid filtrate percentage is decreasing at the fastest rate. This process of decreasing fluid filtrate contamination is referred to as sample clean up. Later, the pumped fluid contains less contamination and the fluid filtrate percentage decreases at a slower rate. Mullins, et. al. published paper on curve fitting of a sample's absorbance versus time to monitor clean up in real time, entitled “Real Time Determination of Filtrate Contamination During Openhole Wireline Sampling by Optical Spectroscopy,” SPWLA, 41st Annual Meeting, Dallas, Tex., June, 2000. The U.S. Pat. Nos. 6,274,865 and 6,350,986 also discuss such curve fitting.
In this paper, Mullins et al. assume that the rate of sample cleanup as measured by observing optical density progresses as t−5/12 where t is time. This clean up rate is based on empirical experience in the Gulf of Mexico and elsewhere. However, Mullins et al. also states that, for extended pumping durations, that the sample cleanup rate for shallow invasion progresses as t−1/3 and that the cleanup rate for deeper invasions progresses as t−2/3. Clearly, an assumption of a sample clean rate of t−5/12 can be rigid and inapplicable to real time situations. Moreover, using time as a fitting parameter necessarily assumes a constant pumping rate. Another problem with monitoring sample clean up over time by looking at optical absorption over time is that sand particles and other particulates can cause considerable scattering, which causes the absorption values measured over time to “jump” and appear noisy. Thus, there is a need for a more flexible model regarding the estimation of formation cleanup based on fluid properties and characteristics for downhole pumping in real time.